Magnetic tape libraries continue to be a key storage tier in data storage infrastructure. In a typical magnetic tape library, there can be hundreds to thousands of tape drives. In order to be able to use the magnetic tape library most efficiently and effectively, it is desired that each tape drive be maintained in peak performance to record and retrieve data for as long a period as possible. For purposes of maintaining the tape drive in such peak performance, it is recognized that the tape head in the tape drive is one of the most critical components that affects performance.
During use of the tape drive, the tape head of the tape drive is configured to be in direct contact with tape from a tape cartridge so that data can be written on and read from the tape as the tape moves across the tape head at high speed. This movement of the tape across the tape head creates friction, while also allowing dust and other particles from the surface of the tape to hone the tape head and collect and build a layer of contaminants, e.g., dust, debris, etc., on the surface of the tape head. Over time, this build-up of the layer of dust, debris, etc. on the surface of the tape head creates excessive separation (also sometimes referred to herein equivalently and alternatively as “spacing loss” or “increased spacing”) between the tape head and the tape. As a result of this excessive separation between the tape head and the tape, the transmission of data between the tape and the tape head begins to degrade until such point that the tape drive is eventually unusable due to an intolerable level of bit errors.
FIG. 1 is a graphical illustration of a theoretical relationship between bit error rate and signal-to-noise ratio for a prior art tape drive, i.e. for a theoretical tape drive. More specifically, FIG. 1 illustrates a theoretical curve 100 that demonstrates such a theoretical relationship, with bit error rate (BER) represented along the Y-axis, and the signal-to-noise ratio (SNR) represented along the X-axis. As shown in the theoretical curve 100 illustrated in FIG. 1, as the SNR of the tape drive increases, the BER decreases.
It is appreciated that BER can also be calculated from SNR in terms of a departure curve. As utilized herein, a “departure curve” is a theoretical curve where BER can be calculated from SNR using random independent errors, also called a complementary error function (“erfc”). For a given channel model such as PR4, the BER can be defined as,BER=K/2×erfc(sqrt(SNR/2)), for some constant K>=1  (Equation 1)
Additionally, FIG. 2 is a graphical illustration of a representative relationship between bit error rate and signal-to-noise ratio for an actual prior art tape drive. In particular, FIG. 2 illustrates a series of actual curves 202 that demonstrate such an actual relationship, with bit error rate (BER) again represented along the Y-axis, and the signal-to-noise ratio (SNR) again represented along the X-axis. Further, as shown in FIG. 2, the X-axis can be used additionally or alternatively to represent spacing loss (or excessive separation) between the tape head and the tape calculated according to the Wallace Equation in magnetic recording theory, with the value of the spacing loss decreasing (going from high-to-low) with movement from left-to-right in the graph.
As further illustrated in FIG. 2, the actual relationship between BER and SNR (or spacing loss) in an actual tape drive reaches a floor level for BER as the SNR increases or the spacing loss decreases to a certain point. Stated in another manner, as opposed to the theoretical curve 100 illustrated in FIG. 1, it is recognized and seen in a real tape drive that the realized BER only decreases to a certain level (i.e. there will always be some errors) regardless of how high the SNR gets (or how low the spacing loss gets). Additionally, as noted in FIG. 2, the floor of the actual curve 202 can vary greatly due to many factors that are seen within the environment in which the tape drive is operating as well as within the tape drive itself. Thus, FIG. 2 illustrates three actual curves, i.e. a first actual curve 202A, a second actual curve 202B, and a third actual curve 202C, that each have a different floor value for how low the BER actually goes. FIG. 2 further shows that regardless of the particular floor for the BER, each of the actual curves 202A-202C demonstrates that tape drive (and tape head) performance essentially degrades together once the SNR gets sufficiently low, or the spacing loss gets sufficiently high.
Also shown in FIG. 2 is a representative operation point 204 (illustrated as a vertical line that crosses each of the actual curves 202A, 202B, 202C). The representative operation point 204 illustrated in FIG. 2 indicates that the tape drive at such operation point includes a good tape head that is operating with a reduced spacing loss and corresponding low BER. Stated in another manner, at the representative operation point 204 illustrated in FIG. 2, the actual curves 202A, 202B, 202C are all effectively at their floors as far as the BER. Horizontal arrow 206, which extends to the left away from the representative operation point 204, indicates the change of performance as the SNR decreases and/or the spacing loss increases. However, because the actual representative operation point 204 is along the flat section of the curve 202A, 202B, 202C, even with a fairly sizable increase in spacing loss, the impact on the BER at this point is still very small.
To prevent those problems which may be caused by excessive separation between the tape head and the tape, the standard approach is to periodically clean the tape head with a cleaning cartridge. In various applications, the cleaning cartridge uses abrasive tape to clean the tape head. In particular, once a cleaning cartridge is loaded into a tape drive, the more abrasive tape in the cleaning cartridge moves across the tape head and contacts the tape head. Consequently, the abrasive tape in the cleaning cartridge scrapes away the build-up of the layer of dust, debris, etc. that has been created on the tape head such that the excessive separation between the tape head and the tape is reduced. Sometimes it can take multiple uses of the cleaning cartridge to fully eliminate the excessive separation between the tape head and the tape. Unfortunately, excessive use of the cleaning cartridge can generate surface scratches and create pole tip recession, which is an unrecoverable permanent separation between the tape and the tape head (or sensor).
Thus, it is appreciated that to maintain longevity of the tape drive and the tape head with higher performance, an operator must vigilantly avoid the two undesirable extremes within the tape drive: excessive separation between the tape head and the tape, and pole tip recession. In order to best avoid such undesirable extremes, it is critical to only apply the cleaning process at the right time and in the correct amount. Currently, that is very difficult in the field operation mode for at least a few reasons, as noted below.
First, the stain build-up process is a complicated nonlinear process that depends on the media type, usage model, and environmental conditions. Thus, the time and the number of cleanings required can be hard to predict. For example, the recommended cleaning period in the specifications of a tape drive, as well as cleaning signals (e.g., in tape_alerts log page) that may be built into the tape drive, often result in cleaning operations that occur too early and too often (which can prematurely lead to pole tip recession) or too late and not often enough (which can result in hard error failure).
Second, the tape drive adaptive channel is very capable of coping with some level of spacing loss. In particular, before the excessive separation between the tape head and the tape reaches a certain critical value (or pivotal point), overall performance of the tape drive may show no change or only insignificant impact during periods of excessive separation between the tape head and the tape. This occurs when the spacing loss (or SNR) effectively corresponds to the floor level for BER, e.g., such as shown along each curve 202A, 202B, 202C in FIG. 2. Unfortunately, once the excessive separation reaches and/or exceeds a pivotal point (i.e. where the actual curve 202A, 202B, 202C starts to move upward fairly rapidly as you move along the actual curve 202A, 202B, 202C from right-to-left), only a relatively small increase in the separation between the tape head and the tape can cause extreme performance degradation.
Third, in a laboratory setting, spacing between the tape head and the tape can be directly measured or indirectly implied through advanced instruments in a controlled environment with a specific setup. However, in field operations, the tape drive may have only a few performance related logs available. Additionally, it is further understood that changes in performance of the tape drive can also be caused by many other factors such as differences in drive, media, or environmental conditions such as temperature, humidity etc. Each of these other factors can have a similar or even greater magnitude of influence on the performance of the tape drive. Any performance changes observed in the tape drive can come from any or all of those factors, and it can be hard to differentiate which portion of performance degradation is due to which cause. Thus, it is difficult to accurately discern when performance of a cleaning operation is truly warranted in such conditions.